fix(federation): Fix federation invite system messages

[nickvergessen]

☑️ Resolves

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

Step	Screenshot
🏚️ Before
With only backend changed
🏠 After

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

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🛠️ API Checklist

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not possible
• 📘 API documentation in docs/ has been updated or is not required
• 🔖 Capability is added or not needed

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Steps to produce

1. Enable federation on your instance occ config:app:set spreed federation_enabled --value yes
2. Use the OCS viewer app https://github.com/provokateurin/ocs_api_viewer and invite your own user to a conversation (adjust the IP to whatever you use to open your own Nextcloud)
   

[Antreesy]

Tested, seems to work